This invention relates to a potshell for an electolytic cell for the electrolytic reduction of aluminum, the potshell including a bottom plate, side walls and end walls of steel plates, and such elements being surrounded by a reinforcing structure.
Experience has shown that in electrolytic cells, or pots, which are operated at high amperages, electromagnetic forces are produced which can disturb operations. These disturbances usually start to appear around currents of 40-50 kA. When even higher currents are used, for example 100-200 kA, the difficulties can become so great that it is impossible to achieve satisfactory operational conditions unless certain, and usually costly, design provisions are made. These electromagnetic forces occur when the current passes through magnetic fields in the electrolytic cell, and they cause waves, bubbles and other movements in the bath and the metal. As the difference in the specific gravity of the molten bath and the underlying molten metal is very small, this can result in considerable variations in the distance between the anode and the molten metal. To prevent the molten metal from touching the anode, thus causing a partial short circuit, the distance between the anode and the cathode has to be increased. This results in a larger voltage drop between the anode and the cathode, hotter pot operation, and higher power consumption for each kilogram of metal produced.
Horizontal electric currents in the metal pad, in conjunction with the vertical magnetic fields, give rise to electromagnetic forces in the longitudinal direction of the pot. To limit the horizontal current components, and thus reduce the most powerful movements in the metal and bath, it is usual practice to arrange for there to be formed an internal coating of frozen bath along the carbon side lining of the pot. This coating, or side freeze, has an electrical insulating effect, and thus limits the horizontal current components. This side freeze is produced by not making the side lining too thick and by not having any thermal insulation between the side lining and the steel shell of the pot. A further advantage of the side freeze is that it protects the side lining from attack by the bath. Nevertheless, it frequently occurs that the molten material penetrates through the side linings with highly adverse consequences and heavy costs. This penetration of the side lining is primarily caused by high operating temperatures, either in connection with the anode effect or on account of poor operating conditions which can by partly due to powerful electromagnetic forces.